The US government is planning to call an American, possibly one of the 22 Navy Seals involved in the Abbottabad raid that killed Osama bin Laden, to give evidence at the trial of Bradley Manning about how he discovered digital material later revealed to contain WikiLeaks disclosures, a military court heard on Tuesday.

Prosecutors intend to bring to the witness stand an anonymous man they are calling "John Doe" who would testify how he entered a room in the al-Qaida leader's hideout in Pakistan, grabbed three items of digital media and removed it. Later, four separate files of information were off-loaded with WikiLeaks contents on them.

The testimony would be used, the prosecution said, to show that Bin Laden had actively sought access to the material Manning had passed to WikiLeaks. That in turn would provide supporting evidence for the most serious charge against the soldier – that he had "aided the enemy".

Ashden Fein, the lead prosecution lawyer, told a pre-trial hearing at Fort Meade in Maryland that the individual who grabbed the digital items, as well as five other witnesses who subsequently handled it in Afghanistan and the US, would be called to show how the WikiLeaks disclosures were used by al-Qaida. "This information was requested by Osama bin Laden; a member of al-Qaida went and got the information and gave it to Bin Laden," Fein said.

In an intense afternoon of legal argument, it was also revealed on Tuesday that Manning has written a personal statement of about 35 pages in which he seeks to explain to the court why he transferred such a massive trove of confidential state documents to the anti-secrecy site. On Thursday the soldier is due to enter a dialogue with the judge presiding over the case, Colonel Denise Lind, in which he is expected to plead guilty to having been the source of the WikiLeaks dump.

The statement was written by Manning in person and hand-typed by him. Discussion in court indicated that in it he makes a declaration of the motives that led him to want to pass information to WikiLeaks – making the account a possibly seminal document.

Lind said that she would decide overnight whether to allow Manning to read out the document in court on Thursday. She insisted that the statement had to have the soldier's signature erased so that it would not be a sworn document – following prosecution protests that they would not be able to cross-examine him on the content of his speech.

The prosecution's desire to call a Navy Seal prompted intense legal argument between Fein and Manning's main defence lawyer David Coombs. Fein said the testimony would be crucial to the government case that Manning "aided the enemy" – a charge that carries the maximum penalty of life in military custody with no chance of parole.

The witness would be produced at an off-site location, Fein said, hinting at the extraordinary security measures that would have to be taken to secure the event. The court would be moved away from Fort Meade to an undisclosed location for the occasion.

By showing that Bin Laden personally asked for, and received, four files' worth of the WikiLeaks material supplied by Manning, Fein said, the prosecution would prove one element of the first charge it has preferred against the soldier – that he "knowingly gave intelligence to the enemy through indirect means".

Coombs countered that whether or not al-Qaida received the leaked material was irrelevant to the charge of "aiding the enemy". What mattered was what Manning thought he was doing at the time he contacted WikiLeaks, not what became of the material he transmitted.

"The key here is: at the time of the offence of giving it to WikiLeaks, what was his knowledge at that point. Actual receipt [of the intelligence] is not relevant to that," Coombs said.

One episode that Manning discusses in his personal statement is an investigation he conducted while working as an intelligence analyst outside Baghdad in 2009. He describes the incident in the web chats he had with the hacker Adrian Lamo that ultimately led to Manning's downfall after Lamo informed on him to military authorities.

In the web logs, Manning says that he was asked to monitor 15 Iraqi detainees being held captive by the Iraqi federal police. He was ordered to find out who the "bad guys" were among the 15 but when he investigated he found that they were in fact peaceful and benign political critics of the government seeking to expose official corruption.

He writes in the logs: "i immediately took that information and *ran* to the officer to explain what was going on… he didn't want to hear any of it… he told me to shut up and explain how we could assist the FPs in finding *MORE* detainee."

That sense of moral outrage is understood to feature in his personal statement as one of his motivating factors in turning to WikiLeaks.

On the day that Bradley Manning was sentenced to 35 years in prison, this Guardian film from 2011 investigates the soldier's early years, and also asks why the US military sent Manning to Iraq, when those training him warned that the 'runt that everyone bullied' should not be sent on active duty